Protonation-coupled redox reactions in planar antiaromatic meso-pentafluorophenyl-substituted o-phenylene-bridged annulated rosarins

Abstract

Proton-coupled electron transfer (PCET) processes are among the most important phenomena that control a variety of chemical and biological transformations. Although extensively studied in a variety of natural systems and discrete metal complexes, PCET mechanisms are less well codified in the case of purely organic compounds. Here we report that a planar β,β'-phenylene-bridged hexaphyrin (1.0.1.0.1.0), a 24 π-electron antiaromatic species termed rosarin, displays unique redox reactivity on protonation. Specifically, treatment with acid (for example, HI) yields a 26 π-electron aromatic triprotonated monocationic species that is produced spontaneously via an intermediate-but stable-25 π-electron non-aromatic triprotonated monoradical dication. This latter species is also produced on treatment of the original 24 π-electron antiaromatic starting material with HCl or HBr. The stepwise nature of the proton-coupled reduction observed in the planar rosarin stands in marked contrast to that seen for non-annulated rosarins and other ostensibly antiaromatic expanded porphyrinoids.

Excitonically Coupled Cyclic BF2 Arrays of Calix[8]‐ and Calix[16]phyrin as Near‐IR‐Chromophores

“…The 1 HNMR spectrum of calix [16]phyrin 5 recorded in CD 2 Cl 2 at room temperature shows aset of NH-signals at 9.8 and 9.7 ppm that corresponds to at otal of eight NH-protons as expected for 5.Asinglet at 6.0 ppm accounts for the four meso-sp 3 -CH protons.T he 11 BNMR spectrum was characterized by the presence of ab road signal at 2.2 ppm. The 19 FNMR spectrum also shows ab road signal at À140.3 ppm corresponding to the F-atom of the BF 2 subunits,rather than the split signal seen in the case of 4.Signals corresponding to the ortho, para,a nd meta CF fluorine atoms appeared at À142.7 (o), À143.2 (o), À153.4 (p)a nd À160.6 (m)ppm, respectively.O nt his basis,w ec onclude that system 5 is relatively symmetric in solution.…”

“…In contrast to the above,t he absorption spectrum of calix [16]phyrin 5 is characterized by the presence of three excitonically coupled bands at 697 nm (allowed), 737 nm (allowed), and 776 nm (slightly allowed). Them olar extinction coefficient for the formal tetramer 5 is larger than those for the monomer 3 (by 2.35-fold) and dimer 4 (by 1.31-fold), as calculated at the l max of the systems in question.…”

“…to what is typically seen in condensations involving bipyrroles and aromatic aldehydes,i ncluding pentafluorobenzaldehyde, no rosarin formation was observed under the reaction conditions. [16] Precursor 2 was then converted to its corresponding BODIPY complex 3 by treating with BF 3 •OEt 2 in the presence of triethylamine in accord with standard protocols. [15] Single crystal X-ray structural analyses of precursors 2 and 3 revealed near-planar structures (Supporting Information, Figures S25 and S26).…”

“…Further insights into the molecular structure of calix- [16]phyrin 5 came from single-crystal X-ray structural analyses of crystals grown through the slow diffusion of hexanes into ad ichloroethane solution of 5. [17] In the solid state, 5 exists as athree-dimensional box-like structure wherein four bis-naphthobipyrrolylmethene subunits are connected through four sp 3 -hybridized meso carbon atoms.T he pentafluorophenyl groups are oriented towards the macrocyclic cavity.Onthe other hand, the four meso-mesityl groups point toward the outside of the cavity (Figure 2).…”

“…With such considerations in mind, we have now prepared two giant cyclic BODIPY systems,n amely calix[8]phyrin-(1.0.1.0.1.0.1.0) (4)and calix [16]phyrin-(1.0.1.0.1.0.1.0.1.0.1.0. 1.0.1.0) (5).…”